1. Field of the Invention
The present invention relates generally to optical imaging systems, and more particularly to a catadioptric imaging system which exhibits good performance at high speed, utilizing a single refractive material type best suited for broad-band ultraviolet applications.
2. Brief Description of the Prior Art
In today's technologically oriented society, a demand has developed for ultra-accurate precision optical instruments and imaging systems. These systems are used in a multitude of applications and must often operate effectively and efficiently over a relatively broad spectral range. In order to accommodate optical functionality, very expensive precision lenses are often used in a variety of complex and functionally limited combinations. The success of these systems has been marginal and oftentimes includes a capital commitment which is effectively cost prohibitive.
A fundamental problem arises which prevents an all-refractive optical system from achieving a broad spectral range with good performance at high speed. That problem is secondary and tertiary color, which is a chromatic variation in focus. In order to compensate for this problem, and maintain an essentially constant focus over a broad ultraviolet range, at least three different types of optical glass normally have to be used, each with a special characteristic dispersion property.
Unfortunately, there is only one optimum optical material that transmits well deep into the ultraviolet, and that is fused silica. Other materials which transmit well, such as calcium fluoride and lithium fluoride, have very undesirable physical properties, such as being soft, difficult to polish, expensive, and susceptible to thermal shock. More importantly, the dispersion properties of calcium fluoride and lithium fluoride are not different enough from that of fused silica to be very useful for achromatising a lens design, and not different enough from each other to be very useful for correcting secondary and tertiary color.
In order to produce an operative broad-band deep-ultraviolet, all-refractive design, for a fast speed objective, an extremely large number of lens elements would be required. Even if this arrangement could be achieved, it is doubtful that the focus constancy could be maintained over a really broad spectral range. Additionally, it would be extremely difficult to correct chromatic variation of aberrations over this range.
An alternative to an all-refractive design is a catadioptric design, which utilizes both lenses and mirrors wherein most of the focusing power of the optical system is due to spherical mirrors. This arrangement greatly diminishes the work done by the lenses, and allows easier control of their color effects. Although a design involving only mirrors would be optimal (because it would work equally well at any wavelength), aspheric mirrors are very difficult to make to the requisite accuracy, and spherical mirrors invariably have three major design defects they are not capable of good performance at fast speeds; they have a large amount of obscuration; and, they produce a strongly curved image.
Prior art catadioptric imaging systems provide a partial solution to these problems, however, several critical tradeoffs are made. First, these systems ultimately produce a strongly curved final image, which is an undesirable quality in any optical system. Furthermore, these systems continue to be constructed using lenses formed from multiple refractive material types. As suggested above, this severely limits the broad-band ultraviolet imaging capabilities of the system. Additionally, a further significant design problem with the prior art catadioptric systems is that major obscurations are produced, which effectively reduce the amount of transmitted light amenable to the system in forming a final image.